You really are having a shocker of a time and MIL is not helping.
Of course the kids are not happy at the see saw visitations, but what can you do if HE decided that visitation days were a good time to get drunk?
As for MIL, well perhaps tell her straight, "if she can't be helpful, just be quiet."
All you can do is keep in your reality, for your own sanity and for your kids sake, as your stbxAH is doing enough 'travel between planets' on his own, and if his visitation rights have to be changed, because of his erratic actions, so be it.
God bless